In ethology, risk compensation is an effect whereby individual animals may tend to adjust their behaviour in response to perceived changes in risk. It is seen as self-evident that individuals will tend to behave in a more cautious manner if their perception of risk or danger increases. An other way of stating this is that individuals will behave less cautiously in situations where they feel "safer" or more protected.

There is evidence that such an effect is seen in humans, associated with the use of safety features such as carseat belts and bicycle helmets. The evidence is particularly compelling for the case of antilock braking systems. The existence of this balancing behaviour does not mean an intervention does not work: the effect could be less than, equal to or even more than the true efficacy of the intervention, depending on how well the perceived efficacy matches actual efficacy - and this will differ from individual to individual. It is likely to be least when an intervention is imperceptible and greatest when an intervention is intrusive or conspicuous.

The theory grew largely out of investigations of road safety interventions. It was noted that most interventions had failed to achieve the forecast savings in lives and injuries. Theorists speculated that while the studies demonstrated that the probability of injury given a crash had reduced, the fact that the overall probability of injury was unchanged indicated that there must have been some change in the probability of crashing.

This controversial view was at first strongly resisted but detailed investigation, particularly of the case of compulsory seat belts, caused the theory to become more widely accepted, although it is still resisted by many who support an interventionist approach[How to reference and link to summary or text].

The logical conclusion of this theory has been reached with the shared space initiatives piloted first in Denmark and the Netherlands[original research?]

, and now being copied eleswhere in Europe and North America. Significant safety benefits have been claimed[How to reference and link to summary or text] from the complete removal of street furniture and signage from urban environments, requiring all users to take more care.

This paper was published at a time when Britain was considering a seat belt law, so the Department of Transport commissioned a report into the issue. In the event the report's author, Isles, agreed with Adams' conclusions. The Isles Report was never published officially but a copy was leaked to the Press some years later[1]. The law was duly passed and subsequent investigation showed, as predicted, no associated reduction in fatalities (although there was some reduction due to the simultaneous introduction of evidential breath testing)[2].

Other research has taken groups of drivers including those who did and did not habitually wear seat-belts and, under the guise of testing new types of belt material and measured the effect on driving style in the habitually unbelted. They were found to drive faster and less carefully when belted[How to reference and link to summary or text].

There are at least three studies which show that drivers' response to antilock brakes is to drive faster, follow closer and brake later, accounting for the failure of ABS to result in any measurable improvement in road safety. The following references describe studies in Canada, Denmark and Germany.

Booth's rule #2, coined by skydiving pioneer Bill Booth, states that "The safer skydiving gear becomes, the more chances skydivers will take, in order to keep the fatality rate constant." Even though skydiving equipment has made huge leaps forward in terms of reliability in the past two decades, and safety devices such as AADs have been introduced, the fatality rate has stayed roughly constant since the early 1980s. Fatality Graph. This can largely be attributed to an increase in the popularity of high performance canopies, which fly much faster than traditional parachutes. High speed maneuvers close to the ground have increased the number of landing fatalities in recent years (see graph), even though these jumpers have perfectly functioning parachutes over their heads.

An associated theory is known as risk homeostasis. This extends risk compensation theory somewhat, although in practice the two terms are often used interchangeably.

Wilde illustrates this by reference to the Swedish experience when they changed from right- to left-hand drive in 1967. This was followed by a marked reduction in the traffic fatality rate, but the trend returned to its previous values after about 18 months. Drivers had responded to increased perceived danger by taking more care; as they became accustomed to the new regime, the additional care evaporated.

Risk compensation is now widely accepted, but risk homeostasis, which goes much further, has a much smaller following.